The present invention relates to noise reduction in a system comprising a digitizer and, more particularly, but not exclusively to noise reduction in a system comprising a digitizer associated with a display screen.
U.S. Pat. No. 6,690,156 “Physical Object Location Apparatus and Method and a Platform using the same” assigned to N-trig Ltd, and U.S. patent application Ser. No. 10/649,708 “Transparent Digitizer” also assigned to N-trig Ltd, describe a positioning device capable of detecting multiple physical objects, preferably styluses, located on a flat screen display. One of the preferred embodiments in both patents describes a system built of transparent foils containing a matrix of vertical and horizontal conductors. In one embodiment the stylus includes a passive resonance circuit, which is triggered by an excitation coil that surrounds the foils. The stylus is excited at a predetermined range of frequencies depending on the capacitance and inductance of the resonant circuit. Other embodiments may include a different kind of EM stylus. The exact position of the stylus is determined by processing the signals that are sensed by the matrix of horizontal and vertical conductors.
Existing digitizer systems use several noise removal methods to improve the detection precision. For example the received signal is processed through a band pass filter leaving a window of frequencies including the stylus frequency. The filtered signal may then be passed through a Fourier transform selecting the single frequency of the stylus.
Elements that induce an equal amount of noise on each conductive line regardless of the line location may then be eliminated through the use of differential amplifiers. For example, objects that are far enough from the sensor will have the same effect on all the sensor lines.
There are other examples of noise reduction methods that do not eliminate noise at the stylus frequency.
Using the various prior art systems, much of the noise is removed, but one element of noise necessarily remains because it cannot be identified and filtered out and that is noise that is at the same frequency as the stylus.